The chest x-ray is used for detecting a number of patient conditions and for imaging a range of skeletal and organ structures. Radiographic images of the chest can be useful for detection of lung nodules and other features that indicate lung cancer and other pathologic structures. In clinical applications such as in the Intensive Care Unit (ICU), chest x-rays can have particular value for indicating pneumothorax as well as for tube/line positioning.
The chest region includes a range of tissues, from rib and other bone structures to the lung parenchyma. This can complicate the task of radiographic imaging for the chest region, since the different types of bone and tissue materials have different densities. Optimization techniques for chest imaging require making compromises to provide a suitable signal-to-noise (S/N) ratio and sufficient contrast for soft tissue.
Chest radiographs are used to examine the lung parenchyma, for which tissue/air contrast is an important feature. As indicated in published work either based on Monte Carlo simulations (“A comparison of mono- and poly-energetic x-ray beam performance for radiographic and fluoroscopic imaging,” J. M. Boone et al., Medical Physics, Vol. 21, No. 12, 1994) or based on experimental measurements (“Investigation of optimum X-ray beam tube voltage and filtration for chest radiography with a computed radiography system,” C. S. Moore, The British Journal of Radiology, Vol. 81, 2008), an identified kV (kVp) range for soft-tissue and air contrast, for average-sized adult patients, is 60 to 80 using poly-energetic x-ray beams. However, an x-ray exposure technique that is used for in-room posterior-anterior (PA) view chest radiography specifies 110 kVp to 130 kVp (such as given in Bontrager's Pocket Atlas, “Handbook of Radiographic Positioning and Techniques,” Bontrager Publishing, Inc.). This higher kVp range is used because, in chest images, the bone contrast from the surrounding rib cage is reduced as much as possible to allow better visibility of the underlying tissue. The Monte Carlo simulation described by Boone et al. indicates that, with increasing exposure kVp, bone contrast decreases at a faster rate than soft tissue contrast decreases. Acquiring chest images at higher kVp helps to mitigate the bone contrast while maintaining a reasonable level of soft tissue contrast. However, the contrast of the lung parenchyma may be viewed as some as being less than optimal. This can complicate diagnosis and features may be misinterpreted.
Higher kVp levels for chest imaging relates to increased x-ray scatter. Scatter reduces image detail contrast and increases noise levels, both of which hinder diagnostic accuracy. X-ray anti-scatter grids are frequently used to reduce scattering, but have negative effects. Grids of higher ratios are required at higher energy levels, increasing the amount of incident exposure that is required to compensate the exposure loss, but at the expense of increased patient-absorbed dose.
A further problem relates to the need for imaging both bone and soft tissue in some patients. Studies by Boone et al. indicate that 50 kVp is an optimal setting for bone contrast. However, standard chest exams are performed at higher kVp, typically around 120 kVp, so that rib bone contrast is reduced in the images obtained, with correspondingly reduced bone detail conspicuity for diagnosis. Thus, patients for whom both thoracic bones and lung regions must be examined undergo two separate examinations, one radiograph taken at the 120 kVp level, another taken at 70 kVp. Because multiple views may be required, a patient may need to undergo more than two exposures for a chest exam, one set of exposures optimized for lung fields, the other optimized for thoracic bones. Thus, the need to image at two different kVp levels can directly translate to double or triple the exposure dose to the patient.
Recent work in rib contrast suppression has shown results that could help to diagnose lung nodules (“Improved Detection of Subtle Lung Nodules by Use of Chest Radiographs with Bone Suppression Imaging: Receiver Operating Characteristic Analysis With and Without Localization,” F. Li et al, American Journal of Roentgenology, vol. 196, 2011, and “Performance of Radiologists in Detection of Small Pulmonary Nodules on Chest Radiographs: Effect of Rib Suppression with a Massive-Training Artificial Neural Network,” S. Oda et al, American Journal of Roentgenology, vol. 193, 2009). As noted from the published images, both methods try to suppress the bone conspicuity completely. In reality, the bone suppression algorithm may not work perfectly; part of the rib bones may still not be sufficiently suppressed. The remaining rib edges may appear as fine lines across the lung field and may have appearance that is similar to pneumothorax, causing mis-diagnosis. Applicants have noted that it is desirable to mitigate this problem when rib contrast suppression is applied.
The use of lower energy x-ray photons helps to maximize soft-tissue and bone contrast in chest radiographs, but there can be negative effects if not applied appropriately. Lower energy photons become absorbed quickly by human tissues as the poly-energetic x-ray beam penetrates the patient. The negative impact of absorption is two-fold: 1) potentially increased patient-absorbed dose, and 2) “beam hardening” effects. Beam hardening essentially modifies the x-ray spectrum and reduces the effectiveness of radiation that is otherwise optimized for chest imaging. This effect becomes worse as patient size increases. Thus, lower energy radiation levels are avoided for chest x-ray imaging, even though these levels could provide improved imaging of soft tissues.
Thus, it can be seen that there is a need for improved methods for setting imaging parameters and image processing parameters that provide the optimal soft-tissue and bone contrast for chest radiography using a single x-ray exposure, and selectively present the anatomical information based on diagnostic purposes at the optimal patient dose efficiency.